The removal of suspended solids and heavy metal ions from the sewage is of vital importance to improving the water quality of the sewage. In general, unless appropriate chemical agents are added to sewage, contaminants in the sewage, such as suspended solids and heavy metal ions, will be uniformly distributed in the sewage or will be settled extremely slowly, and, as a result, it takes much time separating the contaminants from the sewage. For this reason, conventional sewage treatment methods usually involve the use of appropriate chemical agents whereby suspended solids or the other contaminants in sewage can be deposited in the form of sludge.
To speed up precipitation and enable suspended solids to bind with each other to form compact sludge, a conventional hydroxide precipitation technique works by means of chemicals, such as sodium hydroxide, lime, and magnesium hydroxide. In addition, related techniques employ related procedures and a combination of chemicals so as to be commercialized. Nonetheless, the precipitation expediting techniques still have drawbacks in common, that is, overly high water content of the sludge, difficulty in recycling, and low purification efficiency.
As mentioned before, chemicals and mixture precipitation techniques are widely used to remove suspended solids and heavy metal ions from sewage. However, one of their well known drawback is that the water content of sludge is too high (>98%). In order to reduce the water content of sludge, conventional sludge treatment techniques are based on pressure filtration and dehydration procedures. But filter cakes formed as a result of the pressure filtration and dehydration procedures have water content of 50% or more and thus incur high administration costs in the transport and disposal of the filter cakes.
In 1970, Kostenbader, Haines, and the others develop a high density sludge (HDS) process to increase the solid content of sludge to 10˜30% and then perform a pressure filtration and dehydration process to reduce the water content of filter cakes to 30˜50%. The HDS process works by recycling the sludge in part, mixing the recycled sludge and a slaked lime alkaline solution, and delivering the mixture to a water treatment tank such that the mixture reacts with the sludge to render the sludge compact. However, the aforesaid process has major drawbacks as follows:
it requires a reactor for use in sludge recycling and alkalinity adjustment and the reactor occupies a vast piece of land;
the treated filter cakes have high water content;
sewage treatment efficiency will be compromised, if too much sludge is recycled; and
it is impossible to reduce the volume of the sludge with low solid content.
The other related conventional sewage treatment techniques are briefly described below. TW 438725 discloses a sewage treatment technique for removing hazardous ions by agglomeration and sedimentation, wherein an aluminum-containing compound and a polyacrylamide serve as agglomeration agents, and the sediment is reused as an agglomeration agent. TW I228104 discloses a sludge sewage treatment method and device characterized in that copper-containing sewage is treated and converted into precious heavy metal raw materials, and the residues are recycled and reused as an flocculating agent. TW I280951 discloses a fluidized bed process aqueous solution treatment system marked by mixing sewage, a reagent, and reflow sewage fully by a nozzle on a pipeline so as to form crystals. U.S. Pat. No. 7,736,513 discloses a solid-liquid fluidized bed sewage treatment system for use in removing carbon, nitrogen, and phosphorus concurrently, wherein biological fluidized bed technology is applied to the system, such that the sewage treatment system is equipped with two fluidized beds for removing carbon, nitrogen, and phosphorus concurrently.
In conclusion, the prior art fails to address the aforesaid drawbacks, including low cost-effectiveness of sewage treatment equipment in land use, overly high water content of filter cakes, low treatment efficiency, and failure to reduce the volume of sludge.